Nervous System Cells: Structure & Functions

• Nervous System cells' structure and functions are crucial for understanding neural communication and behavior.

Nervous System Overview

• The central nervous system (CNS) includes the brain and spinal cord.
• The peripheral nervous system (PNS) consists of nerves outside the skull and spinal cord.
• Sensory neurons detect changes in the internal/external environment.
• Motor neurons facilitate muscular contraction.
• Interneurons reside between sensory and motor neurons within the CNS.

Neuron Structure

• Soma refers to the cell body.
• Dendrites form the junction (synapse) with the terminal buttons of other neurons.
• Axons are often covered by a myelin sheath and carry action potentials.
• Terminal buttons are the location of neurotransmitter release.
• Axoplasmic transport involves anterograde movement (soma to terminal buttons) via kinesin at 500 mm/day.
• Retrograde transport moves from terminal buttons to the soma, uses dynein, and is about ½ as fast as anterograde.

Internal Neuron Structure

• The membrane acts as the boundary of the cell and contains proteins.
• The cytoskeleton provides the neuron its shape, and is composed of microtubules.
• Cytoplasm pertains to the jellylike fluid containing organelles.
• Within the nucleus, the nucleolus produces ribosomes that synthesize proteins.
• Chromosomes contain genes composed of DNA, and when active, produce mRNA.
• mRNA leaves the nucleus, attaches to ribosomes, and codes for proteins/enzymes.
• The rough endoplasmic reticulum contains ribosomes and produces proteins destined for secretion.
• The smooth endoplasmic reticulum has channels for molecules involved in cellular processes and produces lipid molecules.
• The Golgi apparatus assembles and packages products in a membrane via exocytosis and also produces lysosomes that break down waste products.
• Mitochondria extract energy from nutrients and synthesize adenosine triphosphate (ATP).

Supporting (Glia) Cells

• Glia support cells found for neurons in the central nervous system.
• Astrocytes control the chemical environment around neurons and wrap around neurons and blood vessels.
• Astrocytes aid in neuron nourishment via glucose conversion and glycogen storage and also act as "glue" by holding neurons in place.
• Further, astrocytes surround and isolate synapses to limit neurotransmitter dispersion, remove debris via phagocytosis.
• Oligodendrocytes produce myelin sheath in the CNS, with Nodes of Ranvier being the spaces between myelin tubes on the axon.
• Microglia act as phagocytes and shield the brain from foreign organisms.

Schwann Cells

• Schwann cells produce myelin in the PNS.
• Each myelin segment consists of one Schwann cell.
• Chemical composition of myelin in PNS differs from that of the CNS and is not affected by multiple sclerosis.
• Schwann cells aid after injury in digestion of dead/dying axons and forms tubes for axon regrowth.
• Glial cells in the CNS do not provide the same support for axon regrowth.

Blood-Brain Barrier

• Ehrlich experimented involving injecting blue dye into the blood, which did not dye the CNS to study this barrier.
• Selectively permeable, it regulates the composition inside and outside of neurons.
• Active transport ferries many molecules into the CNS and is more permeable in areas like the area postrema.

Communication Within a Neuron

• Neural communication involves the withdrawal reflex and its inhibition.

Axon Electrical Potentials

• Electrical charge is measured using a microelectrode device.
• Membrane potential refers to the charge difference across the membrane.
• The resting potential is -70 mV.
• Hyperpolarization signifies the inside is more negative than the outside.
• Depolarization signifies the inside is more positive than the outside.
• An action potential is the main electrical event, triggered by the excitation threshold characterized by rapid depolarization followed by hyperpolarization.
• The squid giant axon is used as a research model to understand action potential.

Membrane Potential

• The membrane potential involves the balance of diffusion and electrostatic pressure.
• Diffusion causes molecules to distribute evenly over time, moving from high to low concentration in the absence of barriers.
• Electrostatic pressure involves electrolytes that divide into negatively/positively charged ions.
• Cations exhibit positive charge, while anions exhibit negative charge.
• Electrostatic pressure causes attraction of opposite charges and repulsion of like charges.

Intracellular and Extracellular Ions

• Organic anions (A-) reside inside the cell and cannot pass through the membrane.
• Potassium ions (K+) are concentrated inside, diffuse out, experience electrostatic pressure in, and have little net movement.
• Chloride ions (Cl-) are concentrated outside, diffuse in, experience electrostatic pressure out, and have little net movement.
• Sodium ions (Na+) are concentrated outside, diffuse in, and exhibit electrostatic pressure in.
• The sodium-potassium pump uses high levels of energy, exchanges two K+ in for three Na+ out, maintain low sodium concentrations, and the membrane is also relatively impermeable to Sodium.

Action Potential

• Ion channels are proteins that form pores through the membrane that allow ions to pass into or exit the cell, changing permeability.
• At the excitation threshold, voltage-dependent ion Sodium channels open and Sodium enters, which moves the membrane potential from -70mV to +40mV.
• Voltage-dependent Potassium channels then open and Potassium leaves.
• Sodium channels then close and become refractory at the peak.
• Potassium continues to leave until the membrane potential nears normal and Sodium channels reset.
• As Potassium diffuses, the membrane overshoots the resting potential, but returns to normal.

Conduction of Action Potential

• The all-or-none law dictates an action potential either occurs or doesn't, and once initiated, it transmits to the end of the axon at the same size, even when split.
• Rate law indicates of the rate of firing reveals information strength.
• Saltatory conduction involves passive movement of the action potential under the myelin and regeneration at each node of Ranvier.
• This process expends less energy to maintain ion balance and is faster.

Communication Between Neurons

• Synaptic transmission transfers information from one neuron to another across a synapse.
• This relies on neurotransmitters that produce postsynaptic potentials, and attaches to receptors at binding sites.
• A ligand is a chemical that attaches to a binding site (neurotransmitters are natural ligands).
• These are found on dendrites/dendritic spines and occur on some axons.

Synapse Structure

• The presynaptic membrane is the terminal button before the synapse, while the postsynaptic membrane is the receiving location.
• The synaptic cleft is 20 nm wide and holds extracellular fluid.
• Synaptic vesicles are in terminal buttons, hold neurotransmitters, with transport proteins that fill vesicles.
• Trafficking proteins aid neurotransmitter release/recycling and the release zone is the release location.
• Small vesicles are manufactured by the Golgi apparatus while large vesicles are produced only in the soma.

Neurotransmitter Release

• Omega-structures are synaptic vesicles fused with the membrane.
• Steps for action potential to release include vesicles "docking" against the membrane, and the opening of voltage-dependent Calcium channels after the action potential.
• Calcium then enters the cells and binds with docking proteins, causing them to separate and forming the fusion pore.
• Neurotransmitters are then released to the synaptic cleft.
• Three vesicle pools contains release-read vesicles docked against the presynaptic membrane, while also having less than 1 percent of the vesicles open even with a low axon firing rate.
• Recycling pools accounts for 10-15% of vesicles, while the reserve pool captures the remaining percentage, and are only open with a high axon firing range.
• "Kiss and run" dictates after neurotransmitter release, the undocking and refilling, while "merge and recycle" dictates bulk endocytosis.

Receptor Activation

• The post synaptic receptor receives the neurotransmitter, opening neurotransmitter dependent ion channels.
• Ionotropic receptors causes neurotransmitter binding to open channel and a direct method.
• Metabotropic receptors are an indirect method, requiring energy, and is close to the G protein.
• The G protein generates second messengers can open ion channels, turns on genes, and elicits biochemical changes.
• Metabotropic potential often last longer.

Postsynaptic Potentials

• Excitatory postsynaptic potential (EPSP) involves depolarization, while inhibitory postsynaptic potential (IPSP) involves hyperpolarization.

Termination of Postsynaptic Potentials

• Reuptake stops potentials.
• Enzymatic deactivation involves enzymes destroying neurotransmitters, for example, Acetylcholine (ACh) by acetylcholinesterase (AChE).
• A disease characterized by muscular weakness, which also involves an immune system attacking its own ACh receptors and can be treated by drugs, is called myasthenia gravis.

Neural Integration

• Postsynaptic potentials combine multiple, sometimes contradictory, signals.
• These are performed on by the axon hillock.
• Neural inhibition does not always result in behavioral inhibition.

Autoreceptors

• Autoreceptors respond to the neurotransmitters released, involve metabotropic receptors with inhibitory effect, and help regulate the amount of neurotransmitter released and available for use.

Other Synapse Types

• In axoaxonic synapses, the amount of neurotransmitter is altered via presynaptic inhibition/facilitation.
• Dendrodendritic synapses have regulatory functions.
• Gap junctions are electrical synapses, ions flowing between cells, and are more common in invertebrates.

Non-Synaptic Communication

• Nonsynaptic chemical communication involves neuromodulators that are released by neurons in larger amounts compared to neurotransmitters, and generally are peptides (amino acid chains).
• Hormones that are released by the endocrine system, distributed through the bloodstream, target cells with specialized receptors, and can be classified as peptide or steroid structure.
• Peptide hormones activate metabotropic receptors, while small and fat-soluble steroid hormones bind to receptors, altering protein production.

Explore the structure and functions of nervous system cells, including sensory, motor, and interneurons. Learn about key components such as the soma, dendrites, axons, and terminal buttons. Understand axoplasmic transport mechanisms.